Ever wonder why?
The journal Organic Behavior and the Human Decision Process says it's just human nature.
When you aren't hungry, you'll think in terms of healthful snacks -- fruits and vegetables. But when hunger hits, when you're ready to eat right now, that's when you want the unhealthy snacks.
That's also why you have a kitchen filled with baby carrots and oranges, and you're standing on tiptoes looking for the crumbs from an old bag of tortilla chips.